Electrolytic apparatus and process for removing trace metals



July 22, 969 J. N. MALQNEY, JR., ET AL 3,457,152

ELECTROLYTIC APPARATUS ANDPROCESS FOR REMOVVING TRACE METALS Filed Nov. 50, 1964 INVENTORS .11M N. MALONEY JR.

CHARLES R. CAMPBELL ROBERT JOHNSON United States Patent 3,457,152 ELECTROLYTIC APPARATUS AND PROCESS FOR REMOVING TRACE METALS .lim N. Maloney, Jr., Charles R. Campbell, and Robert Johnson, Pensacola, Fla., assignors to Monsanto Company, St. Louis, Mo., a corporation of Delaware Filed Nov. 30, 1964, Ser. No. 414,675 Int. Cl. B01k 1/00 U.S. Cl. 204-131 9 Claims ABSTRACT OF THE DISCLOSURE Trace quantities of metals can be efliciently removed from an aqueous solution by subjecting the solution to direct electric current in the presence of at least one anode and a lead shot cathode. For use in such a process, the invention also provides an electrolytic apparatus comprising a liquid-containing means having at least one anode and a lead shot cathode, said apparatus being adapted for passage of direct electric current between said anode and said cathode. The process is specifically applicable in the production of adiponitrile by electrohydrodimerization of acrylonitrile.

It has been demonstrated recently that certain organic materials may be dimerized electrolytically. In general, electrolytic dimerization of a desired organic compound may be performed in solution in a cathode compartment of an electrolytic cell having an anode and cathode compartment separated by an ion-permeable membrane. The anode and cathode compartments of such an electrolytic cell contain an anode and cathode, respectively, and upon the application of direct electric current to the anode and cathode, dimerization of the organic compound takes place in good yield in the liquid catholyte solution. The catholyte solution generally comprises an aqueous solution of a salt which increases the solubility of the organic compound and its produced dimer in water.

One such electrohydrodimerization of commercial signicance is the production of adiponitrile, an organic intermediate of known commercial importance, from acrylonitrile, and in long-term continuous operation of a process for the electrohydrodimerization of acrylonitrile, recovery of product and unconverted acrylonitrile from the catholyte solution with recirculation of this aqueous salt solution electrolyte is necessary to make the process economically and commercially feasible. During the longterm continuous operation of such an electrohydrodimerization process, it has been found that the yield of adiponitrile product decreases and the formation of unwanted by-products increases. Efforts to prevent this by the removal of deposited organic solids within the electrolytic cell, filtration of the circulated catholyte and selection of optimum operating conditions have not been successful to maintain product yields at initial high levels and by-product formation at initial low levels.

After much work it has been determined that trace residual quantities of extraneous metals such as copper, nickel, silver, and other metals plate or deposit on the cathode surface in the electrolytic cell, and this cathode fouling by the depositing of metals on the cathode surface was found to correspond with the undesired lowering of product yield and increasing formation of unwanted by-products.

An object of this invention is to provide a process and apparatus for preventing cathode fouling in an electrohydrodimerization process.

Another object of this invention is to provide a process and apparatus for removing trace quantities of metals from aqueous electrolytes used in electrohydrodimerization reaction processes.

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A further object of this invention is to provide a process and apparatus for removing trace quantities of metals from liquid solutions.

These and other objects of this invention will become apparent from the following description of the process and apparatus, which process and apparatus are described and dened with particularity in the claims.

The objects of this invention are accomplished by a new and novel process and apparatus for the electrolytic removal of trace quantities of metal from liquid solutions.

In the accompanying drawing, FIG. 1 is a sectional elevation of an eletrolytic apparatus in accordance with this invention. The illustrated embodiment of the invention comprises tank 10 having bottom 13 and top 14 connected with each other by adjustable spacer bolts 15 which are adjusted and supported by nuts 16 and 17. Gasket 18 is provided at the top end and bottom end of tank 10 between top 14 and tank 10 and bottom 13 and tank 10 for hydraulic seal. Tank 10 may be constructed of glass or a noncontaminating metal such as stainless steel and it is clear to those skilled in the art that other supporting arrangements for the top and bottom or other unitary conligurations for tank 10, top 14, and bottom 13 are clearly within the scope of this invention. When tank 10 and its supporting parts are constructed of a noncontaminating metal such as stainless steel, the metal parts may be electrically connected to the cathode voltage to eliminate the possibility of corrosion of the tank equipment and the subsequent contamination of the aqueous `salt solution being treated therein.

Bottom 13 is provided with liquid inlet 11 and top 14 is provided with liquid outlet 12. Although the direction of flow of liquid through tank 10 is not critical, design of tank 10 and liquid inlet and outlet 11 and 12, respectively, should be such as to provide substantially maximum residence time and uniform distribution of the liquid to be treated within tank 10. As is well known in the art, suitable means may be provided (not shown) for the removal of any generated gas or gases within tank 10 so that substantially all the volume may be utilized for liquid.

Anode 19 is located vertically substantially within the center of tank 10 and surrounded by hydraulically permeable spacer 20. The anode material has little eilect upon the operation of the cell and materials well known in the art for anodes such as carbon and platinum plated titanium as well as many others may be used with great success for the anode. A platinum plated anode has an advantage over a carbon anode of not causing minor contamination of an aqueous salt solution being treated with small particles of carbon. Particulate cathode 21 surrounds hydraulically permeable spacer 20 and substantially completely iills the remaining portion of tank 10. Hydraulically permeable spacer 20 may be constructed or formed from any noncontaminating plastic or metal material or screening such as polyethylene, polypropylene, or stainless steel and others, and the quantity and the size of openings 26 in spacer 26 should be such as to permit maximum flow of liquid therethrough and the passage of no particulate cathode 21. The diameter of spacer 20 is not critical and in general should be maintained at a minimum dependent upon the diameter of anode 19. Spacer 20 should have an inside diameter suicient to provide an annular space between anode 19 and the inside of spacer 20 large enough to prevent electrical shorting between the anode and particulate cathode and to permit the escape of generated gas, if any, as is well known in the art.

Particulate cathode 21 rests upon and is in electrical Contact with cathode connector 22. Cathode inlet support screen 24 prevents cathode 21 from entering inlet 11 and insulates the cathode therefrom. Any suitable metal having a high hydrogen over-voltage may be used for particulate cathode 21 and cathode connector plate 22. In one embodiment designed for the removal of trace quantities of silver and other metal ions which may contaminate an aqueous electrolyte salt solutionlead shot may be used as particulate cathode 21 and lead plate may be used as cathode connector plate 22. Other metals possessing similar hydrogen over-voltage such as zinc, cadmium, and others may be used, either directly or as a plating material, for both particulate cathode 21 and cathode connector 22. It is clear to those skilled in the art that the type of metal suitable for particulate cathode 21 and cathode connector 22. It is clear to those skilled in the art that the type of metal suitable for particulate cathode and cathode connector also depends upon the pI-I of the aqueous salt solution being contacted within the apparatus. The size and shape of particulate cathode 21 is not critical; however, whenever possible, configurations should be chosen to provide substantially maximum cathode surface area within a given volume tank while permitting substantially maximum liquid ow at low pressure drop through the tank.

Insulating gasket 23 may be provided to insulate cathode connector plate 22 from bottom 13, if desired, and saturated calomel electrode may be provided through top 14 within tank 10 to measure the electrical potential between the liquid within tank 10 and the cathode for reference and current control, if desired. If saturated calomel electrode is provided, care should be taken so as to have saturated calomel electrode not in contact with particulate cathode 21 within tank 10, but in a liquid space above particulate cathode.

In operation, an aqueous salt solution having trace quantities of metal impurity or impurities in a concentration up to 100 parts per million and higher concentrations may be treated in accordance with the apparatus and process of this invention by pumping the metal containing aqueous solution up through particulate cathode, by means of inlet 11 and outlet 12 in the embodiment shown in FIG. 1, at a volumetric rate selected to permit a desired residence time, and direct electric current is passed through the apparatus, by means of anode 19 and cathode connector plate 22 in the embodiment shown, to maintain a desired cathodic potential on particulate cathode 21.

Aqueous salt solutions which may be purified in accordance with the process and apparatus of this invention comprise aqueous solutions of organic and inorganic salts which have cations that are not deposited at a potential less negative than the impurity metal ion. Aqueous salt solutions suitable as feed stock for this process and apparatus include aqueous salt solutions of alkali metals, alkaline earths, organic quaternary ammonium salts, and organic amine salts. Particular aqueous organic salt solutions which have been puriied successfully in accordance with the apparatus and process of this invention include tetramethylammonium toluene sulfonate solution, tetraethylammonium ethyl sulfate solution, tetraethylammonium benzene sulfonate solution, and many others.

The concentration of the salt in aqueous solution to be purified may be varied over a wide range. The upper limit of the concentration is set by the solubility of the salt in water and by the increased viscosity of a high concentration aqueous solution which may lower the rate of diffusion of the reducible ion. The lower limit of the concentration of a salt in aqueous solution is set by the conductivity of the aqueous salt solution which must be sufficient to support a plating current without producing an impractical voltage drop across the anode and particulate cathode of an apparatus.

In a preferred application concerning the electrohydrodimerization of acrylonitrile to adiponitrile wherein an aqueous solution of a quaternary alkyl ammonium alkyl sulfate or sulfonate Such as tetramcthylammonium toluene sulfonate or tetraethylammonium ethyl sulfate, may be used as catholyte, a salt solution having a concentration of LlO-% may be purified in accordance with this invention. This concentration is most desirable because operation of the apparatus and process of this invention with the aqueous solution in this salt concentration range will not require additional concentration adjustment of the aqueous salt solution after purification and prior to its use as catholyte in an electrohydrodimerization cell.

The pH of a salt solution to be purified may be both greater than and less than 7 depending upon the metal ion to be removed and the materials of construction of the apparatus. In a preferred embodiment for the purilication of an aqueous salt solution for use as a catholyte in an electrohydrodimerization process, a pH of 7 or above is preferred. The elfect of lowering the pH of an aqueous salt solution to be purified below 7 is to reduce the cathode voltage required for the discharge of hydrogen ions which discharge may prevent the most effective deposition of metal impurity ions on particulate cathode surfaces. When trace quantities of silver, copper, iron, and other easily deposited metals are to be removed from an aqueous salt solution, the pH of the solution may be lower than when the removal of trace quantities of more difliculty deposited metals such as lead, cadmium, nickel, and others is necessary.

The geometry of the apparatus of this invention is of importance to obtain practical rates of deposition of trace metal impurities present in aqueous salt solutions at parts per million concentration levels. The ratio of the particulate cathode surface area to salt solution volume should be maintained at a practical maximum. Apparatus having a ratio of particulate cathode surface area to available salt solution volume of 10i to 500, with a preferred range of 30 to 100 square centimeters of cathode surface per cubic centimeter of aqueous salt solution volume within the apparatus have been found to be practicable, and in a preferred embodiment, a ratio of 38 square centimeters of particulate cathode surface area per cubic centimeter of aqueous salt solution volume may be obtained using No. 8 lead shot which are spherical in shape and have a diameter of 0.09 inch as particulate cathode.

In a single anode type apparatus such as is disclosed in the embodiment of FIG. l, the annular space between the anode surface and the inside of the tank wall, which is the maximum distance from an anode surface to a particulate cathode surface, is important because of nonuniform current density produced by this type of particulate cathode apparatus. This annular distance may be up to 2.5 inches, and it is preferred that the annular distance be from 1.0 inch to 2.0 inches. In a preferred embodiment, an annular space of 1.5 inches gave excellent rates of deposition of trace quantities of silver and copper from an aqueous organic salt solution, and apparatus with a 1.5 inch annular space may be utilized for large scale commercial equipment.

It is a clear that apparatus having two or more anodes arranged vertically in a spaced relationship with each other is within the scope vof this invention. In multiple anode apparatus the geometric arrangement of the anodes should be such as to provide substantially multiples of the apparatus shown and described in FIG` 1. In such an ernbodiment, the distance between any two anodes should not exceed substantially twice the annular distance described above for a single anode type apparatus so that the distance between any given particulate cathode surface and an anode will not exceed an annular distance of 2.5 inches.

It is clear to those skilled in the art that annular distances greater than that specified above for both single and multiple anode apparatus will not make the apparatus inoperable per se but may permit the existence of areas in the annular space, which is in excess of that speciiied, which are substantially dead electrically. These substantially dead areas will be of no value to the operation of the apparatus and may have the deleterious effect of reducing the eiciency of the process and apparatus for the removal of trace metallic impurities.

In a continuous process, efficiency of the removal of trace quantities of metal may be changed by the residence time and the temperature of the solution being treated. Residence time is defined as the liquid volume of the apparatus surrounding particulate catalyst divided Vby the flow rate of an aqueous solution to the apparatus. In general, the greater the residence time and the higher the temperature of the aqueous salt solution being purified the greater the efficiency of the removal of trace quantities of metal at given voltages and amperages for an apparatus. It has been found that a residence time of 10 seconds is adequate; however, a residence time of at least 30 seconds is preferred for most treatment of most aqueous salt solutions. It is clear, also, that care should be taken to operate at a temperature Ibelow the boiling point of water and the vaporizing point of any of the components of the aqueous solution being purified.

In general, direct electrical current to the anode and cathode of an apparatus should be suflicient to supply adequate voltage to plate or deposit unwanted metals on particulate cathode but not high enough to cause appreciable electrolysis of the water of the aqueous solution with the attending liberation of gaseous hydrogen at particulate cathode surfaces. It has been found that cathode voltages of 0.5 volt to -1.5 volts in reference to a saturated calomel electrode are suitable for the removal of trace quantities of unwanted metals in apparatus having particulate cathode surface area to salt solution volume of 10 to 500 square centimeters per cubic centimeter of liquid and an annular spacing up to 2.5 inches.

The effects of temperatures and residence times on the eiciency of silver removal from an aqueous salt solution in accordance with the process and apparatus of this invention are shown in Example I.

EXAMPLE I An aqueous solution consisting of 35 parts by weight water and 65 parts by weight tetramethylammonium toluene sulfonate was prepared, and to this solution, sufiicient silver was added to obtain a silver concentration of 10 parts per million. The apparatus of the embodiment shown in FIG. 1 having an annular space of 1.5 inches and particulate cathode of No. 8 lead shot having a diameter of .09 inch in a bed depth of 10 inches was used. The apparatus was operated at a cathode voltage of -1.2 volts in reference to a saturated calomel electrode. Five samples of the aqueous solution of tetramethylammonium toluene sulfonate were treated on a continuous basis at varying temperatures and varying residence times, and the silver concentration in the efiluent from the apparatus for each of the samples was measured. Table 1 shows results for samples.

TABLE l Temp. of samopa, Residence time of Percent silver sample, minutes removed 40 0. 5 65 40 l. 0 68 60 0. 5 (i7 60 1. 0 78 G0 2. 0 84 The removal of trace quantities of copper from aqueous salt solutions in accordance with the process of this invention is demonstrated by Example II.

EXAMPLE II A 65% aqueous solution of tetramethylammonium toluene sulfonate was prepared containing 11 parts per million copper. The apparatus of the embodiment shown in FIG. l having an annular space of 1.5 inches and using No. S lead shot having a diameter of 0.09 inch in a bed depth of inches as particulate cathode was used to purify the solution. The prepared aqueous solution was passed through the apparatus at a liquid temperature of 25 C. in a volumetric rate sufficient to provide a residence time of 10 minutes within the apparatus while a cathode voltage of 1.2 volts, in reference to a saturated calomel electrode, was maintained upon the apparatus. Analysis of the effluent treated salt showed a copper concentration therein of 2 parts per million.

EXAMPLE III The apparatus of Examples I and II was used on a continuous basis for controlling the level of silver in an aqueous catholyte solution being circulated continuously through an electrolytic cell operated for the electrohydrodimerization of acrylonitrile to adiponitrile. The catholyte solution was an aqueous solution of tetramethylammonium toluene sulfonate in a concentration of 65 by weight. The apparatus was operated to provide a temperature of 25 C. for the aqueous salt solution within the apparatus and the cathode voltage was maintained at 1.2 volts, in reference to a saturated calomel electrode. Flow to the apparatus was controlled to provide a residence time of 10 minutes therein, and samples of feed salt solution to the apparatus and effluent from the apparatus were taken daily for 8 days and analyzed for silver present therein. Table 2 below reports the results of the analysis for each of the samples in parts per million of silver.

TABLE 2 Silver present in parts per million Feed Effluent The advantages of the electrolytic process and .apparatus of this invention for the removal of trace metal impurities from aqueous salt solutions are many. The apparatus described is capable of producing practical rates of deposition of trace metallic impurities at rates which are not obtainable with previously known electrolytic apparatus. Aqueous salt solutions having concentrations of silver or other metals in a range of 0.5 to 1 part per million or less may be obtained rapidly and economically from the efiluent of the apparatus. The electrolytic process and apparatus of this invention for controlling the concentration of trace quantities of metals permits a markedly increased service life of cathodes in electrohydrodimerization processes. Further, the apparatus and process of this invention permits the purification of commercial quantities of aqueous salt solutions in an economical and efficient means.

As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that we do not limit ourselves to the specific embodiments thereof except as defined in the appended claims.

What is claimed is:

1. An electrolytic apparatus comprising a liquid-containing means havin-g liquid inlet means, liquid outlet means, at least one anode, a particulate cathode, and electrical means connected to said anode and cathode for passage of a direct electric current between said anode and cathode, said cathode comprising a body of lead shot.

2. An electrolytic apparatus as defined in claim 1, in which the cathode has a surface area of from 10 to 500 square centimeters for each cubic centimeter of void space surrounding the cathode within said containing means.

3. An electrolytic apparatus as defined in clairn 1, in which substantially Iall surface area of the cathode is within 2.5 inches of at least one anode.

4. A process for removing trace quantities of metals from an aqueous solution containing said trace quantities which comprises subjecting the aqueous solution to direct electric current in the presence of at least one anode and a particulate cathode, said cathode comprising a body of lead shot.

S. A process as defined in claim 4, in which the aqueous solution is subjected to the direct electric current at a cathode voltage of -0.5 to 1.5 volts in reference to a saturated calomel electrode` 6. A process as defined in claim 4, in which the cathode has a surface area of from l0 to 500 square centimeters for each cubic centimeter of said solution.

7. A process as defined in claim 4, in which substantially all surface area of the cathode is within 2.5 inches of at least one anode.

8. A process as dened in claim -4, in which the aqueous solution contains a quaternary ammonium salt.

9. A process as dcned in claim 8, in which the quaternary ammonium salt is tetramethylammonium toluene sulfonate, tetraethylammonium ethyl sulfate or tetraethylammonium benzene sulfonate.

References Cited UNITED STATES PATENTS 673,452 5/1901 Roberts 204-138 XR 883,170 3/1908 Christy 204-268 1,038,122 9/1912 Hagg 204--272 XR 1,857,224 5/1932 Webber et al. 204-131 2,109,151 2/1938 Krause 204-139 2,563,903 8/1951 Zadra 204-272 3,003,942 10/ 1961 Cedrone 204-272 3,141,841 7/1964 Braithwaite et al. 204-59 XR 3,180,810 4/1965 Pearce et al. 204-276 XR ROBERT K. MlHALEK, Primary Examiner G. KAPLAN, Assistant Examiner U.S. Cl. XR. 

